In the past spectrum analyzers for determining the sinusoidal frequency components of complex radio frequency signals containing several such components vectorially added have consisted primarily of sweep tuned local oscillator-superheterodyne receivers which can only display signals sequentially. Other analyzers have been based on Bragg acousto-optic diffraction cells which have the disadvantage that they cannot be driven over large frequency ranges without introducing harmonic distortions because the acoustic transducer designs must provide for a steered acoustic beam to maintain the Bragg interaction angle over the radio frequency range. The Bragg devices can display several signals simultaneously but cannot maintain fidelity over large frequency ranges.
In the heretofore unrelated art of optical filters, an acoustic-optic tunable filter has recently been developed and reported by S. E. Harris and R. W. Wallace as described beginning at page 744 of Volume 59, Number 6 of the Journal of the Optical Society of America in June 1969. Further details were given by S. E. Harris and S. T. K. Nieh at pages 223-225 of Vol. 17 No. 5 of Applied Physics Letters 1 Sept. 1970 and by the same authors together with D. K. Winston at pages 325 and 326 of Vol. 15 No. 10 of Applied Physics Letters 15 Nov. 1969. This tunable filter employed a phenomenon originally described in detail by R. W. Dixon (IEEE J. Quantum Electron. Q. E. - 3, 85 (1967). Dixon noted that in an appropriately oriented crystal, an incident optical beam of one polarization is diffracted into the orthogonal polarization via its interaction with a colinearly propagating acoustic beam. In order for this phenomenon to occur, the active crystal medium must possess a non-zero element of the photoelastic tensor appropriate to the interaction. The appropriate photoelastic constant depends on such factors as crystal symmetry and whether a longitudinal or transverse acoustic wave is employed. Moreover, for the coupling to be effective along the whole interaction length, it is necessary that the optical and acoustic waves be appropriately phase matched. For a given acoustic frequency the phase matching condition is satisfied over a relatively narrow range of optical wavelength. Hence, only light in this wavelength range will be scattered to the orthogonal polarization.
Harris and Wallace proposed an electronically tunable optical filter using this phenomenom. Their basic idea was to utilize colinear acousto-optic diffraction in an optically anisotropic medium in such a fashion that by changing the frequency of a pure sinusoidal driving acoustic wave, changes were produced in the band of optical frequencies that the filter passed. In their paper they give the specific details for a filter using a crystal of LiNbO.sub.3. The Harris and Nieh paper described a filter using a crystal of CaMoO.sub.4. Both papers note that when an acoustic wave travels in such a crystal, the strain induced change of the refractive index of the medium may diffract a light beam that is incident on the medium. In an isotropic medium, the polarization of the diffracted light is unchanged and the diffraction is particularly strong when the light is incident at the Bragg angle. In an anisotropic medium, for certain orientations, light may be diffracted from one polarization to another. In this case, the condition for particularly strong interaction between the acoustic wave and the light wave is that the sum of the k vectors of the incident light and the acoustic wave equal the k vector of the orthogonally polarized diffracted wave. In their filter a crystal orientation is chosen such that an incident optical signal of one polarization is diffracted into the orthogonal polarization by a colinearly propagating acoustic beam. For a given acoustic frequency only a small range of optical frequencies will satisfy the k vector matching condition and only this small range of frequencies will be cumulatively diffracted into the orthogonal polarization. If the acoustic frequency is changed, the band of optical frequencies which the filter will pass is changed.
In the Harris device the crystal is preceeded by a polarizer through which the light to be filtered is passed before entering the crystal and is followed by an analyzer having its polarization axis orthogonal to that of the polarizer so that only those frequency components of the beam which have been diffracted orthogonally in the crystal will pass through the analyzer. An acoustic transducer supplies to the crystal a constant radio frequency signal of known preselected fixed single frequency to determine the pass band of the filter with respect to an optical light beam of unknown mixed and/or variable frequency components.
The general analytic theory of diffraction of light by ultrasonic waves has been discussed at pages 593 through 610 of a book published in 1970 by the Pergamon Press entitled "Principles of Optics" by M. Born and E. Wolf. The application of this analysis to this phenomenon is presented in the above-referenced papers. This analysis is herein assumed as a basis for the mode of operation of the present invention.